you need about 210mb/sec to do uncompressed 8 bit HD. I am sorry that you are stuck with the XServe RAID - because with 14 drives, it barely does this speed. I have no idea what it does with 8 drives (you do have 8 drives, not 4 - correct) - but if you run AJA Kona System Test, you will see that your internal boot drive on your MAC is faster than 50mb/sec. You could bet better performance with any cheapo FW800 drive.
To do uncompressed HD, you can get any number of SATA, Fibre, or SCSI arrays from companies like Cal Digit, Dulce, Sonnet, Ciprico, and so many others, for a fraction of the price of the XServe RAID (and you should have 14 drives for uncompressed HD on the XServe RAID). But as for your 50mb/sec - well, as has just been suggested, go into RAID ADMIN, and check your CACHE FLUSHING, but this is REAL SLOW, and I doubt that this is a CACHE FLUSHING issue. One of the drives is bogging down the entire array, and it is almost impossible to diagnose a single drive problem on an XServe RAID - the RAID Admin will not test individual drives (and Apple support will be no help - you dont' believe me, but you will see once you call them).
SO, what would I do ? Run AJA Kona System test, select a 4 Gig file size and watch the LED's on the front of the XServe RAID, see if one of the lights stays on SOLID, instead of flashing away like the other drives. The one that is solid is probably a flakey drive, and needs to be yanked out (go ahead, it's RAID 50, you won't lose nothing, and you are SCREWED at 50mb/sec, anyway).